The present invention relates to an electronic control unit for a two-cycle internal combustion engine which performs control of an exhaust control valve provided in such a manner as to open and close an upper part of an exhaust port in order to adjust exhaust timing and control for reversing a rotational direction of the engine in accordance with an instruction for reversal.
In a two-cycle internal combustion engine, it is possible to increase an output of the engine by increasing an intake air amount by using a pulsation of exhaust thereby to increase a charging efficiency. For this reason, in a two-cycle internal combustion engine for which an increase in the output is necessary, by providing an exhaust control valve which opens and closes an upper part of an exhaust port and exhaust control valve control means which controls the exhaust control valve in response to a rotational speed of the engine, a position of the exhaust control valve is controlled between a fully-closed position and a fully-opened position in order to obtain an exhaust pulsation effect from a low-speed region to a high-speed region of the engine.
Incidentally, as a matter of fact, the fully-closed position of the exhaust control valve is a position where exhaust timing is delayed most (a position where the upper part of the exhaust port is completely closed) and is not a position where the whole exhaust port is closed.
As described in the U.S. Pat. No. 5,469,818, for example, there is known a controller of an exhaust control valve, which comprises a motor-driven actuator which operates an exhaust control valve and a valve position sensor which detects a position of the exhaust control valve. In this controller, the actuator is controlled in order to coincide a valve position detected by the valve position sensor with a desired position.
In general, when the upper part of the exhaust port is opened (when the exhaust timing is advanced) by increasing an opening degree of the exhaust control valve, a great exhaust pulsation effect is obtained in the high-speed rotation range and the charging efficiency can be increased, whereby the output of the engine can be increased.
Furthermore, when the upper part of the exhaust port is closed (when exhaust timing is delayed) by displacing the exhaust control valve to the fully-closed position, the great exhaust pulsation effect is obtained in the low-speed rotation range and the charging efficiency can be increased, whereby the output of the engine can be increased in the low-speed range.
In the internal combustion engine provided with the exhaust control valve of this kind, when a condition that the exhaust control valve is kept in the closed position or the opened position continues for a long time, foreign substances, such as carbon contained in the exhaust of the engine and tar formed by mixing unburned gas contained in the exhaust gas with oil, adhere to the valve and it becomes difficult for the valve to move, thus making it difficult to control the valve.
Therefore, in an internal combustion engine provided with an exhaust control valve, foreign substances such as carbon and tar are removed by performing a cleaning operation which involves forcedly moving the exhaust control valve from the fully-closed position to the fully-opened position and the cleaning operation which involves forcedly moving the exhaust control valve from the fully-opened position to the fully-closed position.
In vehicles which put importance on simplicity of the operation, such as a motor scooter and a snowmobile, usually a transmission cannot be provided with gears for backing up and, therefore, it was impossible to perform a backward travel operation with power of an internal combustion engine. Recently, however, with a feature of a two-cycle internal combustion engine which can rotate in both forward and reverse directions, trials have been made to cause not only forward travel, but also backward travel to be performed by use of the engine by performing a reversal control which allows the rotational direction of the internal combustion engine to reverse in accordance with an instruction for reversion.
The reversal control for reversing the rotational direction of an engine can be performed as follows as described in the U.S. Pat. No. 5,036,802, for example.
That is, when an driver performs an operation for giving an instruction for reversal, such as an operation of a reversal switch, after lowering of an inertia force of a piston of the engine by lowering the rotational speed of the engine to a sufficiently low speed which is not more than an idling speed by a misfire of the engine, a cut of fuel, a delay of ignition timing, etc., an explosive force overcoming the inertial force of the piston which has displaced toward a top dead center is generated by igniting the engine at excessively advanced timing and a rotational direction of a crankshaft is reversed by pushing back the piston. Then, after it is ascertained that the rotational direction of the engine is reversed, the engine is ignited at a timing suitable for keeping the operation of the engine in the reversed rotational direction, thereby the operation of the engine is continued in the reversed rotational direction.
In order to certainly perform the reversal control each time in a two-cycle internal combustion engine which is so designed as to perform such reversal control as described above, it is necessary to generate an appropriate combustion pressure in a stable manner when the ignition timing is excessively advanced.
However, in a two-cycle internal combustion engine provided with the exhaust control valve, a compressibility of an air-fuel mixture changes depending on the position of the exhaust control valve. When the compressibility changes, an air-fuel ratio which is most suited for combustion changes. Therefore, the combustion pressure tends to fluctuate according to the change in the position of the exhaust control valve.
In particular, during a cleaning operation, a fluctuation width of combustion pressure increases during a period in which the exhaust control valve is displaced from the fully-closed position to the fully-opened position and during a period in which the exhaust control valve is displaced from the fully-opened position to the fully-closed position.
For this reason, if the cleaning operation of the exhaust control valve happens to take place when the reversal control is being performed, the combustion pressure produced within the cylinder during an ignition operation performed at excessively advanced timing is not stable, thus causing a problem that probability of a failure in reversing the rotational direction increases due to an insufficient combustion pressure.
In particular, in a two-cycle internal combustion engine for which a fuel is supplied by an electronic fuel injection device, it is difficult to finely control an injection amount of the fuel so that an air-fuel ratio of an air-fuel mixture is adapted to a change in the compressibility of air-fuel mixture while the compressibility is changing due to a change in a position of an exhaust control valve. Therefore, the injection amount of the fuel tends to become indefinite while the position of the exhaust control valve is changing during the reversal control. For this reason, the combustion which takes place during an ignition operation performed at excessively advanced timing becomes unstable when the position of the exhaust control valve is changing during the reversal control, and the combustion pressure may sometimes become insufficient. A probability of a failure in reversal control increases due to this insufficient combustion pressure.
Accordingly, an object of the present invention is to lower a probability of failure in control to reverse a rotational direction of a two-cycle internal combustion engine provided with a valve controller which controls a position of an exhaust control valve and a reversal controller which performs control to reverse the rotational direction of the engine.
The above-described object is achieved by providing an electronic control unit for a two-cycle internal combustion engine which comprises: a valve controller, which controls a position of an exhaust control valve provided in such a manner as to open and close an upper part of an exhaust port of the internal combustion engine to adjust an exhaust timing of the two-cycle internal combustion engine; a reversal controller which performs a control to reverse a rotational direction of the internal combustion engine in accordance with a reversal instruction; and valve opening and closing operation inhibition means which inhibits an opening and closing operation of the exhaust control valve while the reversal controller is performing the control to reverse the rotational direction of the internal combustion engine.
As described above, when there is provided means for inhibiting an opening and closing operation of the exhaust control valve while the reversal controller is performing the control to reverse the rotational direction of the reversal controller, it is possible to prevent fluctuations in a combustion pressure generated when an ignition operation is performed at an excessively advanced timing during the reversal control. Therefore, it is possible to lower the probability of the failure in reversing the rotational direction of the engine.
It is preferred that the above-described valve opening and closing operation inhibition means be comprised in such a manner as to fix the exhaust control valve in a fully-closed position while the reversal controller is performing the control to reverse the rotational direction of the internal combustion engine, thereby inhibiting the valve opening and closing operation of the exhaust control valve.
A combustion pressure generated when the engine is ignited reaches a maximum when the exhaust control valve is in a fully-closed position. For this reason, when the position of the exhaust control valve is kept in the fully-closed position during the reversal control as described above, the reversal control can be positively performed by increasing the combustion pressure generated during the reversal control.